In recent years, the design rule for LSI has been progressively adopting more refined pattern of sub-quarter micron order, and as a result the shortening of the wavelength of the exposure light source has been compelled. Thus, the exposure light source has been shifted from g-line (436 nm) and i-line (365 nm) lights of the heretofore commonly used mercury lamp to KrF excimer lasers (248 nm) and ArF excimer lasers (193 nm), and studies have been made to further shift to the EUV exposure technology, wherein EUV (Extreme Ultra Violet) light having a dominant wavelength of 13.5 nm in used.
In manufacturing semiconductor devices such as LSI and super-LSI or in manufacturing a liquid crystal display board, a pattern is made by irradiating a light to a semiconductor wafer or a negative plate for liquid crystal; however, if a dust adheres to a mask for lithography (also simply referred to as “mask”) or a reticle (collectively referred to as “exposure original plate” herein below) used in this pattern creating stage, the dust absorbs light or bends it, causing deformation of a transferred pattern, roughened edges or black stains on a base, and leads to problems of damaged dimensions, poor quality, deformed appearance and the like.
Thus, these works are usually performed in a clean room, but, even in a clean room, it is yet difficult to keep the exposure original plate clean all the time. Hence, in general the exposure light irradiation is conducted only after a pattern-bearing part of the surface of the exposure original plate is sheltered by a pellicle as a dust fender. Under such circumstances, the dust particles do not directly adhere to the surface of the exposure original plate, but only onto the pellicle membrane, and thus by setting the exposure light to focus at the pattern described on the exposure original plate at the time of lithographing, the dust particles on the pellicle membrane do not affect the transferred image of the pattern.
In general, a pellicle is basically built up in a manner such that a pellicle membrane having a high transmittance against the light used in the exposure process is adhered tensely to an upper annular face of a pellicle frame, and an airtight gasket is adhered to the lower annular face of the pellicle frame. The airtight gasket is generally composed of an agglutinant layer. The pellicle membrane is made of cellulose nitrate, cellulose acetate and a fluorine-containing polymer or the like which transmit well such lights that are often used in the light exposure (e.g., g-line [430 nm] and i-line [365 nm] created by mercury lamp, or KrF excimer laser [248 nm], and ArF excimer laser [193 nm]) or in the case of the EUV exposure, a very thin silicon is being studied to make the pellicle membrane.
The pellicle is installed for the purpose of preventing the dust from adhering to the exposure original plate, and the installation is effected as the agglutinant layer of the pellicle is pressed upon the exposure original plate. On this occasion, the pellicle is positioned in a manner such that the pellicle frame entirely surrounds the pattern region formed in the surface of the exposure original plate, so that the pattern region is isolated from the external atmosphere by means of the pellicle so that the dust outside the pellicle cannot reach the pattern region. Thus, a pellicle closed space is created by the pellicle membrane, the pellicle frame and the exposure original plate.
As the pellicle closed space is sealed with the airtight agglutinant layer, it forms an airtight space; hence when the external pressure varies, there occurs a difference in pressure between the inside and the outside of the pellicle with a result that in the case of a pellicle membrane made of a resin such as fluorine-containing polymer, the pellicle membrane is pressed to inflate or deflate. Also, in the case of EUV exposure, the light exposure operation is conducted in vacuum so that an operation chamber of an EUV exposure apparatus is drawn to vacuum each time the mask is entered or removed from the chamber, and if the pellicle membrane is made of an extremely thin rigid silicon film, even a small pressure change is highly liable to break the film.
Accordingly, in order to mitigate the pressure difference across the pellicle membrane, it is a general practice to provide an air passage through the pellicle frame to communicate the inside and outside spaces with each other so that air can pass freely between these spaces. For example, IP Publication 1 describes a pellicle in which an air passage for pressure adjustment is made through a side bar of the pellicle frame and a filter is provided to cover the air passage at the outside wall of the pellicle frame so as to prevent dust particles from entering. Also, IP Publication 2 describes a pellicle for EUV, which comprises a support member having an inner porous part which occupies an area interior to its outer frame, a filter which allows air to permeate through a frame bar of this support member, and a pellicle membrane made of single crystal silicon supported by the porous part of the support member, so that the pellicle membrane does not undergo flections or breakage by pressure changes even when the pellicle is used in vacuum environment.
As described above, a conventional pellicle is commonly provided with an air passage for pressure adjustment and a filter for prevention of particles from entering through this air passage. Also, in such a pellicle, the openings of the air passage are made in the two wall surfaces which are vertical to the upper annular face as well as the lower annular face of the pellicle frame, and also the filter is provided in a manner such that it covers that opening of the air passage which is made in the outside wall of the pellicle frame. The air that is passing a filter loses more of its flow velocity (flow rate) than when it is passing an air passage so that, especially in the case of EUV exposure operation wherein vacuum drawing is conducted and even a slight pressure difference across the pellicle membrane becomes a problem, it is important to increase the area of that opening of the air passage which is covered up with the filter so as to secure a sufficient effective filtering area.